In the measurement and control of liquids flowing at low flow rates a number of problems arise, particularly where the liquid is toxic and/or corrosive. In particular, conventional metering equipment does not permit high accuracy in the measurement and control of liquids flowing at low flow rates, e.g. of the order of 0.1 liter per minute, such as are now in demand in gasoline blending operations, where the demand, on environmental grounds, is for ever lower lead levels in gasoline.
Various processes and apparatus have been disclosed in the art for continuously metering liquids and involving two reservoirs which are charged in turn with the liquid, the one reservoir being filled whilst the other is discharged. For example, in U.K. Pat. No. 1,094,818 an apparatus is disclosed comprising two reservoirs connected in parallel between a supply and a discharge conduit. Reversing valves are provided in the feed and discharge conduits to switch the in-flow and out-flow to and from each reservoir in turn. The flow rate is measured by a conventional flow meter connected between the two reservoirs. Such a system is not sufficiently accurate in the metering and control of liquid flow at the low flow rates required by present day technology, due partly to limitations of accuracy inherent in rotational flow meters, but also due to the fact that measurement is by volume, which is subject to variation with temperature, thereby introducing variations in the accuracy of measurement under conditions of varying temperature, as may occur for example, in continuous gasoline blending operations carried out over a period of time. Also reliance on rotary flow-meters involving moving parts is unsatisfactory due to wear and risk of breakdown. Reliability and elimination of breakdowns is particularly important when handling a toxic or hazardous liquid since exposure of maintenance personnel to the liquid, as for example, during essential repair and maintenance work must be kept to a minimum.
In British Pat. No. 811,462 a metering and flow control system is disclosed primarily for particulate solids, but also allegedly for liquids. In that system the material to be metered is fed at a controllable rate into one of two hoppers mounted in parallel, whilst material fed to the other hopper in a previous feed cycle is discharged at a controllable rate into a common collecting trough. Each of the two hoppers is mounted on or suspended from a weighing device, e.g. a transducer or beam balance, which responds to the weight of material in each hopper, i.e. the one being filled and the one being emptied. Control of supply and discharge of the material to and from each hopper is effected by deliberately imparting a progressive unbalance to the two weighing devices coupled with a pneumatic or electrical control system which seeks to bring the system back into balance by opening or closing valves in the supply and discharge lines to the hopper. Such a system is extremely complex and inherently unsuitable for the measurement and precise control of liquids at low flow rates, because the whole weight of the hopper plus its contents is applied to the balance arm or transducer. Compared with this total weight, the weight variation due to the discharge of a liquid at, for example 0.1 liters per minute, is but a minute fraction of the whole, and it is questionable whether such a system would operate with any degree of accuracy at the extremely low flow rates envisaged by the present invention. Moreover, the requirement that each hopper is movably mounted on or suspended from a weighing device presents considerable structural and functional disadvantages.
Other references disclosing continuous feeding apparatus of the type comprising two reservoirs or hoppers mounted in parallel and into one of which, during the first part of the operating cycle, the material to be metered is fed, whilst the material fed to the other during a previous cycle is being discharged, with reversal of the operations of filling and discharging at the end of each cycle, are disclosed in French Pat. No. 1,203,876 and U.S. Pat. No. 3,690,392.
In French Pat. No. 1,203,876 an apparatus is described for the accurate measurement of a liquid at low flow rates comprising two open cups each mounted on the end of a different balance arm. Each cup can be alternately filled and emptied with the liquid to be metered through a set of electrically operated valves. At the opposite end to the cup, each balance arm has a counter-weight attached to the arm by an electromagnet. As each cup is filled with liquid, the weight of liquid in the full cup overcomes the magnetic force holding the weight at the opposite end of the balance arm, and the balance tips moving the cup from a raised filling position to a lowered emptying position. The tipping of the balance arm actuates the control system to shut the respective filling valve and initiate the emptying cycle, which is achieved by pumping the liquid out of the full cup. Once the cup is empty, the electromagnet on the opposite end of the balance arm is re-energised to attract the counter-weight and cause the balance arm to tip back into the filling position. A chronometer is used to measure the filling and emptying time of each cup, and also a counter is provided for counting the number of filling and emptying operations, i.e. the number of balance movements, for each cup within a given time. From these figures, the total mass flow and the instantaneous flow rate over a given period, e.g. one minute, can be determined.
As will be appreciated such a system involves a series of weighing operations in which a discrete predetermined quantity of liquid is weighed out on each balance in turn and then discharged. A sequence of weighing operations is involved using two balances each with moving parts. The reliability of such a system is therefore open to question, particularly when operated over long periods of time, and of course, the system is not open to ready variation or control of the quantity of liquid being discharged. Moreover, the system operates using open cups, which are inherently unsuited to the handling of toxic materials such as lead tetraalkyls.
U.S. Pat. No. 3,690,392 discloses a bulk feeding apparatus in which two hoppers are filled and emptied in alternating cycles with the material to be metered. The weight of each hopper is taken at the end of each filling operation and added to the cumulative total. The system is designed for handling of bulk materials, particularly bulk solids, to produce rapid accurate results under extreme rugged environmental conditions without special care. The system is clearly not operable on liquids at extremely low flow rates under very exact and closely controlled environmental conditions.